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E39 (1997 - 2003) The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

Every time the season changes between winter and spring and summer and fall my system drops coolant. A noticeable pool on the garage floor. This has been going on for years and the dealer says there is nothing that they can find that is wrong. Just had it checked again.

This car has a new rad, new expansion tank, new cap etc and nothing is leaking. Basically when we go from the really cold to the mild weather or from mild weather to really cold I lose about 300 ml of coolant. I can see it on the garage floor. Once the car issues the "Low Coolant" warning I add BMW coolant mixed with distilled water 40/60. I raise the coolant level to the proper level as measured by the float. I fill it once and only once per season change.

Everything runs fine and the discharge stops until the next season change. I wash the garage floor and everything drys out so I know this isn't an ongoing leak. Once the seasons change the coolant pool reappears and the process repeats.

All parts came from a BMW dealer in Toronto so I assume they are all OEM. This is a very large dealer so I'm just assuming they are BMW parts.

The coolant leak is at the front of the car, centered between passenger and driver side. There is a tray under the rad with a small drain hole, a rectangle about 3/8 inch long and 1/4 inch wide. I see some coolant in this tray. Can't really see any signs of where it is coming from but I going to assume it near bottom of rad.

Not certain if my situation is/was the same, but I just completed the timing chain guides on our '01 X5 4.4i and my new best friend, (an Indy that was recommended by my regular Indy) discovered a small leak in the bypass hose that allows warm water into the heating radiators for the HVAC. I couldn't understand why or where this pool of @ 1/2 pint or so came from...I think that was the problem...we pck it upi on Monday and I'll report back to make sure thatwas the culprit

Golferjohnm raises a good point. If your car has the belly pan in place...a leak from the back of the engine can go undetected due to the leak falling in the pan, then finding the lowest spot in the pan to fall to the garage floor.

If everything or many parts up front has been changed...there are hoses etc that run to the heater core & water/heater valve along the rear of the engine bay. Those hoses will deteriorate over time due to heat and age and could possibly be the culprit...especially a temp change that may cause a hose or band around a hose to expand contract etc.

Strange. The ambient temperature change between seasons is smaller than the coolant temperature change between a hot and cold engine. If there was a leak prompted by temp variations seems to me that it would occur consistently, not just once in Spring and once in the Fall.

Do you drive your car during the Winter or store it? Do you check your coolant level regularly or wait until the warning light comes on?

Temp does not go past 12:00 unless really hot day in summer sitting on really hot highway and then just a little bit.

Does this happen in heating season.... not really but I guess I could answer yes. This happens when the seasons change so in fall I start to use heater and in spring I use it less. I would further clarify this by saying I leave the auto temp set at 21 C and just change the air flow from the vents to be either cool/cold to warm/hot air.

Re the pan. Definitely not from back on engine. The pan I was referring to is right underneath the Rad and there is a air gap between this "drip tray" and engine. The fluid is in the center of this pan, no signs on either side of pan.

Yes I drive this car all year round. No I do not check level, I just wait for light. I will mention again I see this pool under car. I back into garage so under the front end is really noticeable. I see the pool and within a week the light is on.

Just had car in for "Service 2" and the mechanic said they checked and no leaks.

Would a faulty thermostat leak periodically and where would I find this bypass hose?

More info on the Rad service I had a couple of years a go. It was leaking coolant into engine so they replaced gaskets, and hoses and expansion tank. Had a small leak in a small hose, top right of rad and when they replaced it they cracked rad so brand new rad. I was hoping this would be the end of the seasonal leak, it wasn't.

One of the service advisers said if you own a 540i adding coolant was just part of owning this vehicle. I laughed but I beginning to think this is a fact, no a myth.